C#

Usage examples: Visitor isn’t a very common pattern because of its complexity and narrow applicability.

Conceptual Example

This example illustrates the structure of the Visitor design pattern. It focuses on answering these questions:

• What classes does it consist of?

• What roles do these classes play?

• In what way the elements of the pattern are related?

Program.cs: Conceptual example

using System;
using System.Collections.Generic;

namespace RefactoringGuru.DesignPatterns.Visitor.Conceptual
{
    // The Component interface declares an `accept` method that should take the
    // base visitor interface as an argument.
    public interface IComponent
    {
        void Accept(IVisitor visitor);
    }

    // Each Concrete Component must implement the `Accept` method in such a way
    // that it calls the visitor's method corresponding to the component's
    // class.
    public class ConcreteComponentA : IComponent
    {
        // Note that we're calling `VisitConcreteComponentA`, which matches the
        // current class name. This way we let the visitor know the class of the
        // component it works with.
        public void Accept(IVisitor visitor)
        {
            visitor.VisitConcreteComponentA(this);
        }

        // Concrete Components may have special methods that don't exist in
        // their base class or interface. The Visitor is still able to use these
        // methods since it's aware of the component's concrete class.
        public string ExclusiveMethodOfConcreteComponentA()
        {
            return "A";
        }
    }

    public class ConcreteComponentB : IComponent
    {
        // Same here: VisitConcreteComponentB => ConcreteComponentB
        public void Accept(IVisitor visitor)
        {
            visitor.VisitConcreteComponentB(this);
        }

        public string SpecialMethodOfConcreteComponentB()
        {
            return "B";
        }
    }

    // The Visitor Interface declares a set of visiting methods that correspond
    // to component classes. The signature of a visiting method allows the
    // visitor to identify the exact class of the component that it's dealing
    // with.
    public interface IVisitor
    {
        void VisitConcreteComponentA(ConcreteComponentA element);

        void VisitConcreteComponentB(ConcreteComponentB element);
    }

    // Concrete Visitors implement several versions of the same algorithm, which
    // can work with all concrete component classes.
    //
    // You can experience the biggest benefit of the Visitor pattern when using
    // it with a complex object structure, such as a Composite tree. In this
    // case, it might be helpful to store some intermediate state of the
    // algorithm while executing visitor's methods over various objects of the
    // structure.
    class ConcreteVisitor1 : IVisitor
    {
        public void VisitConcreteComponentA(ConcreteComponentA element)
        {
            Console.WriteLine(element.ExclusiveMethodOfConcreteComponentA() + " + ConcreteVisitor1");
        }

        public void VisitConcreteComponentB(ConcreteComponentB element)
        {
            Console.WriteLine(element.SpecialMethodOfConcreteComponentB() + " + ConcreteVisitor1");
        }
    }

    class ConcreteVisitor2 : IVisitor
    {
        public void VisitConcreteComponentA(ConcreteComponentA element)
        {
            Console.WriteLine(element.ExclusiveMethodOfConcreteComponentA() + " + ConcreteVisitor2");
        }

        public void VisitConcreteComponentB(ConcreteComponentB element)
        {
            Console.WriteLine(element.SpecialMethodOfConcreteComponentB() + " + ConcreteVisitor2");
        }
    }

    public class Client
    {
        // The client code can run visitor operations over any set of elements
        // without figuring out their concrete classes. The accept operation
        // directs a call to the appropriate operation in the visitor object.
        public static void ClientCode(List<IComponent> components, IVisitor visitor)
        {
            foreach (var component in components)
            {
                component.Accept(visitor);
            }
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            List<IComponent> components = new List<IComponent>
            {
                new ConcreteComponentA(),
                new ConcreteComponentB()
            };

            Console.WriteLine("The client code works with all visitors via the base Visitor interface:");
            var visitor1 = new ConcreteVisitor1();
            Client.ClientCode(components,visitor1);

            Console.WriteLine();

            Console.WriteLine("It allows the same client code to work with different types of visitors:");
            var visitor2 = new ConcreteVisitor2();
            Client.ClientCode(components, visitor2);
        }
    }
}

Output.txt: Execution result

The client code works with all visitors via the base Visitor interface:
A + ConcreteVisitor1
B + ConcreteVisitor1

It allows the same client code to work with different types of visitors:
A + ConcreteVisitor2
B + ConcreteVisitor2